This invention relates to chemically sensitive sensors such as ISFETs for use in the detection and measurement of chemical properties and a method for the manufacture of header mounted sensors for use in measuring probes. The term "chemically sensitive" as used herein, shall be understood to include sensitivity to ion activity and the presence, concentration and activity of any of a variety of chemical and biochemical substances. The term header is used to mean the structure for physically supporting sensor elements, such as ISFETs, as well as for providing the electrical contacts from the outside to the sensor, such as to the substrate, source and drain regions of an ISFET. Typical header structures are too small for easy handling in industrial applications and are therefore usually mounted in a larger body structure so that the combination makes up what is referred to herein as a probe.
When used as a pH sensor, for example, an ISFET must be mounted on a header of some sort, not only as a support, but also as a means for sealing the ISFET itself, except for the chemically sensitive region, from the electrolyte being measured. This sealing will thus provide environmental interfacing of the ISFET and the ambient electrolyte. The header assembly is then mounted in a probe body structure which is either permanently mounted for immersion in the electrolyte or hand held for immersion.
The electrolytes being measured are normally a very hostile environment for electrical circuit elements. It is, therefore, desirable to have the sensor so constructed as to minimize contact between the associated electrical circuit elements and the electrolyte solution so that the integrity, stability, and reliability of the device is maintained. I have previously solved this problem as it relates to an individual ISFET chip, particularly the source and drain region contacts, by etching holes from the back of the ISFET to the source and drain regions with sidewall isolation provided in those holes and a metallization of the surface of those sidewalls to provide a connection between the source and drain regions and contact pads or areas on the back of the ISFET. The back of the ISFET was then protected by the body of the probe in which the ISFET was mounted. One form for such a structure and a method for manufacturing it is disclosed in U.S. Pat. No. 4,505,799, issued to me on Mar. 19, 1985. That patent is hereby incorporated by reference as a part of the present specification.
As mentioned above, and as shown in FIGS. 3, 4 and 5 of the referenced patent, it is necessary to mount the ISFET chip in a probe assembly which will adapt it for the particular type of measurement intended. In the referenced patent this mounting involves the mounting of the ISFET chip on a header or support member, such as a piece of aluminum oxide or borosilicate glass, and dielectrically isolating the edges of the chip by coating the edges with epoxy. As described, the chip may be bonded to the header or support member by use of epoxy or by anodic bonding if the header is borosilicate glass. The isolation of the sidewalls of the chip and the mounting on the header, when carried out on the individual ISFET chips, as in this patent, is an expensive and time consuming procedure which does not lend itself to mass production at low cost. Furthermore, it would be more desirable to provide an inorganic (dielectric) protection which would be more durable under the adverse conditions of high temperature and high pH. Such protection is difficult or uneconomical to apply at the chip stage of manufacture, however.
It is an object of this invention to provide a method for easily and inexpensively manufacturing header mounted solid state chemically sensitive sensors, such as ISFET structures useful for pH probe assemblies, which will have the sidewalls of the silicon chip on which the sensors are formed dielectrically isolated from the solution to be tested and which will provide contacts for the sensor, such as for the source and drain regions of an ISFET, as well as a substrate contact, in a manner such that those contacts will not be exposed to the solution to be tested when the header is mounted in a probe assembly.